A two cycle engine is rather simple in construction and commonly omits various valves as essential in a four (stroke) cycle engine to complete a cycle of operation in a cylinder having a combustion chamber. In the two cycle engine, to accomplish a two stroke cycle of operation, a fuel gas in the form of an air and fuel gaseous mixture is not directly admitted or introduced into the combustion chamber. But, tightly closed, a crank case is associated with a cylinder to accept the fuel gas from a carbureter via a fuel intake port. A reed valve or rotary valve in this port, the only valve that is incorporated in the two cycle engine, is adjusted to control the rate of flow of the fuel gas taken into the crank chamber. The opening and closing actions of the fuel gas intake port as well as an exhaust port for discharging a spent fuel or waste gas and a scavenging port for refreshing a fuel (an air and fuel mixture) from the crank chamber into the combustion chamber in the cylinder, are all governed by the two stroke cyclic movements of a piston in the cylinder.
Thus, the piston, that is slidably received in the cylinder and designed to reciprocate in it between a pair of its dead points, commonly called an upper dead point and a lower dead point, serves to open and close the fuel intake port, the scavenging port and the exhaust port. The piston in the cylinder is typically designed in the form of a dome providing a hollow space with its top side or side adjacent to the combustion chamber closed and its down or opposite side open to the closed space that is defined by the inside of the cylinder and the crank chamber.
In operation of such a conventional two cycle engine, the piston moving from a first dead point in one of the two direction (hereafter called "moving upwards" or "ascending" from its "lower dead point" for the purpose of the brevity of expression) creates a suction or negative pressure in the hollow space therein that causes the fuel intake port to open, permitting a fuel gas (air and fuel mixture) to be sucked or taken into the crank chamber. At the same time the piston ascending in the cylinder also causes a previous fuel (air and fuel mixture) to be compressed in the combustion chamber above the piston in the cylinder.
The piston ascends or continues to move in that one direction until it reaches its second dead point (hereafter "upper dead point") at which point of time the fuel gas compressed in the combustion chamber is explosively fired by the ignition of a spark plug disposed therein to drive the piston to move in the other direction from the second dead point (hereinafter called "descending" or "moving downwards" from its "upper dead point").
The piston descending from its upper dead point first opens the exhaust port to permit a spent fuel or waste gas in the combustion chamber to be instantaneously forced out and discharged therethrough. When the scavenging port is thereafter opened to the combustion chamber or a space above the piston with the piston descending in the cylinder, the refreshing fuel gas (air and fuel mixture) being compressed in the crank chamber is forced to flow from the latter via a scavenging passage and the scavenging port and is admitted into the space now under suction lying above the piston to fill the combustion chamber in the cylinder.
The scavenging port is closed with the piston ascending after it reaches its lower dead point, whereas the exhaust port is allowed to remain open until the fuel gas in the space above the piston or the combustion comber has started to be compressed with the piston before it reaches its upper dead point and until after the scavenging port is closed.
Therefore, with a conventional two cycle engine in which a fuel gas (an air and fuel mixture) is not directly taken in the combustion chamber as in a four cycle engine but its intake and compression must be effected in a single stroke, it has now been observed that the intake, especially of air in the combustion chamber is insufficient and it is because of this that a failure to gain enough torque output of the engine is unavoidable; even an explosive combustion every stroke or cycle cannot yield an output doubling the output which a four cycle engine can normally provide.
Indeed, with the exhaust port held by the piston to remain open until after it has commenced compressing a fuel gas in the combustion chamber, a considerable amount of the fuel gas unburnt is emitted to the environment through this port from the engine over its scavenging and, especially, compression operating time interval. Such a significant loss of fuel in an operation of the traditional two cycle engine has left it of a poor fuel economy, an insufficient gain of torque performance and also deficient in the capability to restrain the environmental pollution.